The art of flight control law design is couched in a trade-off between maneuverability and stability. While in theory, optimal responsiveness of an aircraft can be provided by permitting a pilot to operate each control surface and throttle to the maximum actuation range in every situation, in practice this is generally not desirable, in part because of unsafe combinations of actions given the interrelation of the control elements, environment, and instantaneous orientation and motion of the aircraft, and because of the very high workload involved in controlling multiple effectors concurrently to retain stability of the aircraft. It is generally difficult to achieve a balance that permits a pilot to safely and productively operate the aircraft while retaining stability in a wide variety of conditions, without requiring a very high workload. One known technique for breaking this trade-off is by defining different response modes that generally provide adequate workload and adequate maneuverability for the pilot in respective ranges of operating conditions.
For rotorcraft, in some operating conditions high levels of stability augmentation are desired (and even specified by design standards), and low levels of stability augmentation are desired for other conditions. For example, in degraded visual environments modes with high levels of stability are typically selected, whereas low levels of stability augmentation are typically specified for flight in conditions where visual cueing is good. These modes have contrary objectives: in response to a change in command, the stability augmented response modes are slower to respond, but minimize the effect of disturbances from interaction with an environment, and, in contrast, high mobility modes provide greater responsiveness but are more greatly affected by the environment. Minimally augmented response modes allow the pilot to be more aggressive, and more precise with the handling of the aircraft, whereas the highly augmented modes provide security associated with higher stability, and reduce the pilot's workload, as less effort is needed to continuously balance and counterbalance controls to stabilize the aircraft. Usually in poorly cued environments, the pilot will not typically attempt to manoeuvre the aircraft aggressively owing to the lack of visual references. Highly augmented modes may also be desired when photographing, lifting or manipulating objects, for example.
Response modes of rotorcraft control systems, known as response types, determine how the rotorcraft responds to a given pilot input. For modern rotorcraft, lateral and longitudinal axis response types in increasing levels of augmentation (stability) are: Rate Damped (RD), in which the angular velocities of the fuselage (roll and pitch) are proportional to pilot inceptor (stick) displacement; Attitude Command/Attitude Hold (ACAH), in which the angle of the fuselage (roll and pitch) is proportional to stick displacement; and Translational Rate Command (TRC), in which aircraft velocity (airspeed, or groundspeed) is proportional to stick displacement.
Mode transitions in modern rotorcraft, including fly-by-wire rotorcraft, are typically manually selected by the pilot or armed for conditional transition for example, in response to a function of airspeed and (in some cases) on control stick position. For example, in the CH-47F the control system response type will change automatically from ACAH to TRC at speeds below 10 knots provided the pilot has armed the TRC control mode. However, it is possible for the pilot to fly with an ACAH response type from forward flight into the hover by not arming the TRC control mode. If the pilot opts not to arm TRC, then the pilot risks that upon achieving the hover that the visual conditions may degrade owing to the rotor downwash, as can commonly happen in desert/dusty, or loose/light snow conditions (a state known as ‘brownout’, or ‘whiteout’ respectively), or during fog or in low-light conditions. In such a case, the pilots would then have to make an additional action to arm and engage the TRC response type upon encountering the brownout or whiteout condition, which focuses pilot attention on managing the control system, rather than flying the aircraft, at a very critical time. Conversely, if the pilot opts to employ TRC in a well cued environment, it is possible to encounter situations that may warrant a desired immediate switch in control response to ACAH, or Rate Command. For example, if during a landing task in a hostile area, a weapon carrying enemy soldier is spotted, then the pilot might prefer to have the more aggressive performance characteristics of ACAH or Rate Command over the stability of TRC; once again requiring a discrete mode switch that focuses pilot effort on managing the control system rather than flying the aircraft.
While it may be frequently useful to provide the operator with the ability to manually switch between these modes, and the decision about modes may often be non-problematic for pilots, there are situations where transitions between these modes is difficult or requires the pilot's attention at an inopportune moment. These cueing conditions do not typically correspond uniformly with groundspeed, altitude, stick position, or other aircraft or sensor indications, and thus automatic triggering in response to such indications may be inconvenient or unhelpful or even dangerous. Layers of triggers for multiple transitions are complex, and require greater management of the control system by the pilot. Armed transitions are triggered in response to the specified condition, and are generally only as useful as the prediction made at the time of arming the transition, that once the specified condition is met, the transition will be desired. It is exceedingly difficult to arm for unexpected transitions, and these are generally when they are most needed.
U.S. Pat. No. 7,433,765 appears to disclose a fly-by-wire (FBW) static longitudinal stability system which provides an unobtrusive airspeed hold function that reacts to pilot control inputs and the measured states of aircraft, to engage smoothly without any explicit mode selection by the pilot when the aircraft is in a trimmed, non-accelerating state and disengages smoothly when the pilot commands an aircraft pitch or yaw manoeuvre. This system is limited to engaging or disengaging a single mode. The engagement or disengagement is responsive to pilot control inputs and the measured states of the aircraft: i.e. the attitude, and motion of the aircraft determines the mode.
U.S. Pat. No. 4,645,141 discloses an automatic flight control system which allows the pilot to manually control a helicopter by displacing the control stick and automatically return to hover position hold or a hover velocity hold upon a natural release of the control stick. The system of U.S. Pat. No. 4,645,141 seems to have a very limited capability of controlling the flight based on the sensed behaviour of the control stick, with automatic control engaging only upon “natural release” of the stick and being limited to hover position or hover velocity control.
Other air, marine and aerospace craft that use multiple response modes for turning command signals into control output for actuating control means that permit feedback-based redirection of the craft are equally susceptible to conflicting rationale for different response modes in different conditions. Such craft may include: blimps and dirigibles, fixed wing aircraft, submarines, ships (docking systems), unmanned aerial vehicles, unmanned underwater vehicles, landing craft, orbital vessels for docking with other orbiters, and the like.
In an unrelated field, WO 01121981 to McIndoe et al. teaches an apparatus and method for operating a continuously variable transmission, such as a toroidal drive type transmission of a land motor vehicle. The continuously variable transmission is selectively operated in either a torque control strategy or a ratio controlled strategy, depending on the operating conditions of the vehicle, and thereby benefits from the advantageous aspects of both the torque and ratio control strategies, while avoiding the disadvantageous aspects of both strategies. Specifically, the transition from the torque control strategy to the ratio control strategy (and vice versa) can be accomplished by simultaneously calculating the control\pressures that would result from operation in both the torque and ratio control strategies, and further assigning a weighted value to each of such calculated control pressures based upon current operating conditions. The summation of such weighted values provides a composite control signal that facilitates a smooth transition between the two control strategies.
This equipment controls a single pneumatic device that controls transmission in a motor vehicle, which is not a craft, and although negative feedback is used to damp changes, this is not a feedback control loop that uses information from sensors, let alone sensors that relate to the orientation and/or motion of the vehicle. The control strategies are not response modes, as they do not involve any control law.
In the present field, there is a need for new methods for transitioning between response types of rotorcraft that permits pilots to transition seamlessly between modes having different degrees of augmentation. Preferably such transitioning is provided in an intuitive manner that requires minimal training for pilots.